Freeze or dehydrate: only two options for the survival of subzero temperatures in the arctic enchytraeid Fridericia ratzeli.

Hygrophilic soil animals, like enchytraeids, overwintering in frozen soil are unlikely to base their cold tolerance on supercooling of body fluids. It seems more likely that they will either freeze due to inoculative freezing, or dehydrate and adjust their body fluid melting point to ambient temperature as has been shown for earthworm cocoons and Collembola. In the present study we tested this hypothesis by exposing field-collected adult Fridericia ratzeli from Disko, West Greenland, to freezing temperatures under various moisture regimes. When cooled at -1 degrees C min(-1) under dry conditions F. ratzeli had a mean temperature of crystallisation ( T(c)) of -5.8 degrees C. However, when exposed to temperatures above standard T(c) for 22 h, at -4 degrees C, most individuals (90%, n= 30) remained unfrozen. Slow cooling from -1 degrees C to -6 degrees C in vials where the air was in equilibrium with the vapour pressure of ice resulted in freezing in about 65% of the individuals. These individuals maintained a normal body water content of 2.7-3.0 mg mg(-1) dry weight and had body fluid melting points of about -0.5 degrees C with little or no change due to freezing. About 35% of the individuals dehydrated drastically to below 1.1 mg mg(-1) dry weight at -6 degrees C, and consequently had lowered their body fluid melting point to ca. -6 degrees C at this time. Survival was high in both frozen and dehydrated animals at -6 degrees C, about 60%. Approximately 25% of the animals (both frozen and dehydrated individuals) had elevated glucose concentrations, but the mean glucose concentration was not increased to any great extent in any group due to cold exposure. The desiccating potential of ice was simulated using aqueous NaCl solutions at 0 degrees C. Water loss and survival in this experiment were in good agreement with results from freezing experiments. The influence of soil moisture on survival and tendency to dehydrate was also evaluated. However, soil moisture ranging between 0.74 g g(-1) and 1.15 g g(-1) dry soil did not result in any significant differences in survival or frequency of dehydrated animals even though the apparent wetness and structure of the soil was clearly different in these moisture contents.